1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission mounted on a vehicle, more specifically, to a system providing hydraulic control of a so-called clutch-to-clutch (change-over) speed change operation wherein a predetermined speed stage is obtained by engaging some frictional engagement elements while disengaging other frictional engagement elements, and particularly to a system providing hydraulic control of a clutch-to-clutch speed change operation at the time of an up-shift.
2. Description of the Related Art
In general, vehicle operating (running) conditions, include a power-on state in which an accelerator pedal is depressed during travel so that driving power is transmitted from an engine to wheels, a power-off state in which the accelerator pedal is released during travel so that driving power is not transmitted from the engine to wheels, high-speed driving, high-torque driving, low-torque driving, low-speed driving and the like. Speed change of the automatic transmission is controlled in accordance with the operating conditions experienced during travel.
One approach to hydraulic control of the above-mentioned clutch-to-clutch (change-over) speed change is disclosed in Japanese Patent Application Laid-Open No. 307524/1994. In the disclosed conventional apparatus, disengagement of pre-shift frictional engagement elements is by reduction of engagement force in accordance with a plurality of preset, pre-shift control stages arranged in a continuous series. Likewise, engagement of post-shift frictional engagement elements is set to a plurality of post-shift control stages arranged in a continuous series. Depending on change in rotational speed of the input shaft, the pre-shift control stages and the post-shift control stages are executed in parallel such that a speed change from the pre-shift to the post-shift is executed. Upon execution of the pre-and post-shift control stages, all the stages or some selected stages are executed continuously in the corresponding order to achieve a shift-up speed change in the power-off state or in the power-on state, smoothly and without delay.
As described above, the hydraulic control of the related art detects the operating condition of the vehicle, e.g. power-on or power-off, and selects an appropriate control stage based on the result of that detection. Thus, a control logic for selecting the above-mentioned appropriate control stage is necessary, which requires a corresponding memory capacity.